Electroluminescent display unit including discharge path



Filed NOV. 15, 1957 Nov. 3, 1 970 B. A. 5m- 3,538, 0

ELECTROLUMINESCENT DISPLAY UNIT INCLUDING DISCHARGE PATH Gem 901 2 Sheets-Sheet 1 coumoz.

MEANS 1 2 mvzm'on EURTOIV A. BABE AGENT US. Cl. 315169 United States Patent O 3,538,380 ELECTROLUMINESCENT DISPLAY UNIT INCLUDING DISCHARGE PATH Burton A. Babb, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation, Nutley, N..I., a corporation of Maryland Filed Nov. 15, 1967, Ser. No. 685,240 Int. Cl. H!) 37/00, 39/00 13 Claims ABSTRACT OF THE DISCLOSURE An electroluminescent display system utilizing DC. voltage sources and simple switching apparatus to apply an A.C. excitation voltage to selected ones of an array of electroluminescent cells. Each cell is provided with its own discharge path.

FIELD OF THE INVENTION This invention relates to electroluminescent display systems and more particularly to an electroluminescent display unit which provides an A.C. excitation signal for electroluminescent cells and wherein each cell is provided witha discharge path.

DESCRIPTION OF THE PRIOR ART Many systems utilizing electroluminescent cells in a matrix-type array are presently known in the art. In the majority of these systems, rows and columns of conductors are orthogonally arranged, the cross-points of the conductors being insulated from each other. At each cross point, there is generally connected an electroluminescent cell in series with a rectifying element, the rectifying element isolating the cells from reverse voltages applied to the conductors when energizing other selected cells. Typical examples of this type of array are shown and described in US. Pat. Nos. 2,818,531 and 2,774,813.

One important disadvantage of these prior art systems is that a leaky electroluminescent cell must be used for proper operation since no means are provided for discharging an electroluminescent cell once it is lit. Therefore the inherent leakage of the cell must be relied upon for discharging, thus slowing down the rate at which the system may be operated. in order to increase the operational speed of such prior art systems, a very leaky electroluminescent cell must be used, thereby decreasing the efiiciency of the system. It is further pointed out that the electroluminescent cells shown in the above two US. patents cannot be excited with an A.C. voltage since the rectifying element, as used in these prior art circuit configurations, will not pass an A.C. signal.

Electroluminescent cells are non-polarized devices having a breakdown voltage rating which cannot safely be exceeded in either the positive or negative direction. The light output from these cells is proportional to the frequency and the peak-to-peak amplitude of the voltage applied thereto. If such a cell is utilized in a DC. system (i.e., only unidirectional voltages being used), the maximum peak-to-peak voltage that can safely be applied thereto is equal to just under the maximum permissible breakdown voltage. When utilized in an A.C. system, however, the maximum peak-to-peak voltage that may be applied across the electroluminescent cell is slightly less than double the maximum breakdown voltage when the input excitation signal is centered about a zero reference level. Also, it is pointed out that the light output of an electroluminescent cell is not directly proportional to the peak-to-peak voltage applied thereto, and that doubling the peak-to-peak applied voltage substantially more than doubles the light output from the cell. Therefore, it is 3,538,389 Patented Nov. 3, 1970 desirable that any given electroluminescent cell be operated in an A.C. mode in order that the maximum amount of light be safely obtained therefrom and that better contrast ratios be obtained.

SUMMARY OF THE 1N VENTION Therefore, the main object of this invention is to provide a novel system for driving selected ones of an array of electroluminescent cells with an A.C. excitation voltage.

It is a further object of this invention to provide A.C. operation of selected ones of an array of electroluminescent cells and still provide isolation of unselected cells from reverse voltages applied to other selected cells in the array by means of rectifying elements.

It is yet another object of this invention to provide an improved array of electroluminescent cells wherein each cell is provided with a discharge path, thereby making it unnecessary to rely solely upon the inherent leakage of the cell itself for discharging.

An electroluminescent display unit according to this invention includes an electroluminescent cell having a discharging means coupled to one terminal thereof and a unidirectional current conducting means connected to the junction of the cell and discharging means. A capacitive element is coupled between another terminal of the electroluminescent cell and another terminal of the dis charging means. Further provided is a first voltage source coupled to said another terminal of the discharging means, a second voltage source coupled to said another terminal of the electroluminescent cell and a third voltage source coupled to another terminal of the unidirectional current conducting means.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration of a portion of a typical matrix-type electroluminescent array according to this invention;

FIGS. 2A through E are an illustration of the waveforms appearing at designated points in the array of FIG. 1; and

FIGS. 3A through E are an illustration of waveforms appearing at designated points in the array of FIG. 1 when an electroluminescent cell is maintained dark.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is shown a typical 3 x 2 matrix array of electroluminescent cells according to this invention. It should be clear that other arrays could also be used within the spirit of this invention. The array of FIG. 1 comprises first column conductors 1 and 2, second column conductors 3 and 4 and first, second and third row conductors 5, 6 and 7. Coupled between the first column conductors 1 and 2 is a capacitor 8 which is common to the column defined by conductors 1 and 2. A similar capacitor 9 is coupled between column conductors 3 and 4. The value of capacitors 8 and 9 is approximately 10 times the efiective capacitance of the electroluminescent cells in the column. Coupling conductors 1 and 3 to a voltage source V are rectifying elements 10 and 11, respectively. These rectifiers are common to their respective columns. Further coupled to column conductor 1 is an electroluminescent cell 12, the other terminal of electroluminescent cell 12 being coupled to the second column conductor 2 via a resistor 13. A rectifying element 14 couples the row conductor 5 to the junction of the electroluminescent cell 12 and the resistor 13. The elements 12, 13 and 14 comprise a cross-point element which is denoted generally as 15 in FIG. 1. The other cross-point elements illustrated in FIG. 1 are generally denoted as elements 16-20 and include elements similar to elements 12, 13 and 14 within the respective dashedline blocks. All of the cross-point elements operate in the same manner and therefore only cross-point element 15 will be discussed in detail herein. Coupled between a voltage source +V and conductors 2, 4, 5, 6 and 7 are resistors 21-25, respectively. Switching transistors 26-30 are also coupled to conductors 2, 4, 5, 6 and 7, respectively for selectively coupling these conductors to ground potential for selectively operating the electroluminescent cells at the cross-points 15-20. The base electrodes of transistors 26-30 are coupled to a control means 31 which selectively switches these transistors on and off in order to selectively couple the said conductors to ground potential. Control means 31 is of such a nature that one ordinarily skilled in the art could design same within the spirit of this invention. Therefore, a more detailed description thereof is not deemed necessary for a proper understanding of this invention. Resistors 21-25 are the load resistors for transistors 26-30, respectively.

It is pointed out that the matrix of FIG. 1 is merely an illustration of a portion of an array of electroluminescent cells and that the capacitors 8 and 9 and the rectifying elements 10 and 11 are common to all of the cross-point elements in their respective columns.

All of the cross-point elements in each column comprise elements similar to the electroluminescent cell 12, the resistor 13 and the rectifying element 14 which appear at cross-point 15. The values of the capacitors 8 and 9 are large with respect to the total capacitance of all of the electroluminescent cells in their respective columns. A satisfactory ratio is capacitors 8 and 9 having at capacitance value of approximately 10 times the total capacitance of all of the electroluminescent cells in a column.

Referring to FIG. 2 in conjunction with the circuit of FIG. 1, the operation of one of the cross-point elements 15 of the display system will be described in more detail. The waveforms shown in FIGS. 2A, 2B, 2C and 2D are the voltages appearing at the points labeled A, B, C and D, respectively in FIG. 1. FIG. 2B shows the voltage across electroluminescent cell 12 (FIG. 2C minus FIG. 2D). FIGS. 2A and 2B are the voltages applied to conductors 5 and 2, respectively, in order to energize electroluminescent cell 12. In the particular embodiment being described the magnitude of voltage V is one-half the magnitude of voltage V, but it is understood that these voltages are merely illustrative and that the invention is not limited to a system wherein the voltages comply with this limitation. The waveform of FIG. 2C appears at the junction of the electroluminescent cell 12 and the resistor 13 and the waveform of FIG. 2D represents the voltage appearing on the column conductor 1.

In the inoperative state the transistors 26 and 28 are off, thereby applying a constant DC. voltage V to conductors 2 and 5 via respective resistors 21 and 23. The voltage appearing at point C during this period of time is +V. The voltage at point D is equal to 3V/2 (i.e., the voltage V via capacitor 8 plus the voltage V/2 via diode 10). The voltage across the electroluminescent cell 12 is therefore V/ 2 which is equivalent to the voltage at point C minus the voltage at point D. When it is desired to operate the electroluminescent cell 12 at cross-point 15, at time t transistor 26 is turned on by control means 31 to apply ground potential to conductor 2 (see FIG. 2B). At this time, the voltage across electroluminescent cell 12 jumps abruptly from V/Z to +V/ 2, the voltage at point C remains at +V and the voltage at point D goes to V/2. Most of this voltage change appears across cell 12 since the capacitor 8 is large with respect to the total capacitance of all the electroluminescent cells in any column. At time 1 the transistor 26 is turned off and the voltage V is again applied to conductor 2. This causes the voltage at point C of FIG. 1 to abruptly jump from +V to +2V, the voltage at point C then decaying back to +V with a time constant RC, where R is the resistance value of resistor 13 and C is the effective capacitance of electroluminescent cell 12. Since the voltage across the electroluminescent cell 12 is equal to the voltage at point C minus the voltage at point D, the Voltage thereacross also decays with a time constant RC, but from the voltage -|-V/2 back to the voltage V/2. Note that during the preceding sequence of events the transistor 28 was maintained in off state, thereby applying a constant voltage V to conductor 5.

In order to produce a constant light output from electroluminescent cell 12, the cell must be periodically driven in the same manner as described above. That is, the drive signal of width (t t of FIG. 2A must be periodically applied to the cross-point element 15, such as at times t and t (see FIG. 2A) to produce the periodically repeating AC. voltage across cell 12 as illustrated in FIG. 2B.

Thus, it is seen that the peak-to-peak voltage across the electroluminescent cell 12 during the above described sequence of events is approximately equal to the magnitude of the voltage V. This peak-to-peak voltage of magnitude V is applied to an electroluminescent cell 12 whose maximum voltage rating is only '-V/2. The cell 12 is not damaged since maximum voltage applied thereto never exceeds :V/Z referred to ground potential. In this manner it is possible to obtain more light from any given electroluminescent cell than by operating it in a DC. mode.

The above described cross-point element may be used in matrix applications where it is desired to scan the crosspoint elements of the electroluminescent display and tooperate certain cross-point elements thereof. For example, if it is desired to light a row of cross-point elements the transistor switch associated with the particular row being scanned is turned off and the switching transistors for the columns are sequentially driven off and on by control means 31 to sequentially reproduce the waveform of FIG. 2B. It is also possible to light more than one row simultaneously by operating the appropriate row switching transistors and then scanning the columns. If it is desired not to light certain cross-point elements of a row during the scanning operation, then the row switching transistor must be turned on (to place ground potential on the row conductor) at the time when the drive signal is applied to the column associated With the cross-point element which is to remain unlit. FIG. 3 illustrates the waveforms required to maintain a cell dark when the drive signal is applied to its column. It is believed that the design of a control means 31 to perform the abovedescribed scanning functions may be carried out by one ordinarily skilled in the art within the spirit of this invention and therefore a description of a particular scanning apparatus is not included herein.

The instant invention was reduced to practice as an 8 x 10 matrix (8 rows, 10 columns) and the following component values gave satisfactory results at an operating frequency of approximately 400 frames per second:

Resistors 21-25=47 kilohms Resistors R=2.5 megohms Column capacitors 8 and 9:.025 ,uf.

While I have described above the principles of my invention in connection with specific apparatus it is to be clearly understood that this description is made only by Way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. An electroluminescent display unit comprising:

first, second and third voltage sources;

an electroluminescent cell;

discharging means, one terminal thereof being coupled to one terminal of said electroluminescent cell for discharging said electroluminescent cell after it is lit;

unidirectional current conducting means coupled to the junction of said electroluminescent cell and said discharging means;

capacitive means coupled between another terminal of said electroluminescent cell and another terminal of said discharging means;

means coupling said first voltage source to said another terminal of said discharging means; means coupling said second voltage source to said another terminal of said electroluminescent cell; and

means coupling said third voltage source to another terminal of said unidirectional current conducting means.

2. An electroluminescent display unit according to claim 1, wherein at least one of said voltage sources is a pulsed voltage source.

3. An electroluminescent display unit according to claim 1 wherein said means coupling said second'voltage source to said electroluminescent cell includes a second undirectional current conducting means.

4. An electroluminescent display unit according to claim 1, wherein said discharging means includes a resistor, one end thereof being coupled to one terminal of said electroluminescent cell and the other end thereof being coupled to said capacitive means.

5. An electroluminescent display unit according to claim 2, wherein said first voltage source is a pulsed voltage source.

6. An electroluminescent display unit according to claim 2 wherein said third voltage source is a pulse voltage source.

7. An electroluminescent display unit according to claim 2 wherein said first and third voltage sources are pulsed voltage sources.

8. An electroluminescent display unit according to claim 1 wherein the magnitudes of saidfirst and third voltage sources are substantially equal.

9. An electroluminescent display unit according to claim 8 wherein the magnitude of said second voltage source is approximately one-half the magnitude of said first and third voltage sources.

10. An electroluminescent display unit according to claim 1 wherein said means coupling said first voltage source to said discharging means includes first switching means.

11. An electroluminescent display unit according to claim 1 wherein said means coupling said third voltage source to said unidirectional current conducting means includes second switching means.

12. An electroluminescent display unit according to claim 1 wherein:

said means coupling said first voltage source to said discharging means includes first switching means;

said means coupling said third voltage source to said unidirectional current conducting means includes second switching means; and

further comprising control means coupled to said switching means for selectively operating said first and second switching means.

13. An electroluminescent display system comprising:

first, second and third sources of voltage;

first conducting means including at least one pair of conductors;

second conducting means including at least one conductor;

at least one unidirectional current conducting means coupling said first source of voltage to one conductor of said at least one pair of conductors;

at least one capacitor coupled between thec onductors of said at least one pair of conductors;

a plurality of electroluminescent crosspoints coupled to said first and second conducting means, each said crosspoint including:

an electroluminescent cell coupled to one conductor of said at least one pair of conductors;

discharging means coupling said electroluminescent cell to the other conductor of said at least one pair of conductors; and

a second unidirectional current conducting means coupling said electroluminescent cell to said at least one conductor of said second conducting means; and

means for selectively applying said second source of voltage to said at least one conductor of said second conducting means and for selectively applying said third source of voltage to said other conductor of said at least one pair of conductors to selectively energize said at least one electroluminescent crosspoint.

References Cited UNITED STATES PATENTS 2,651,740 9/1953 Lair 315-167 X 2,774,813 12/ 1956 Livingston 315169 X 2,818,531 12/1957 Peek 315166 3,060,345 10/1962 Sack 315169 3,379,931 4/ 1968 Soldano 315169 3,409,800 11/1968 Myers et a1. 315-169 X 3,409,887 11/1968 Blank 315169 X JOHN W. HUCKERT, Primary Examiner A. J. JAMES, Assistant Examiner US. Cl. X.R. 

